Baranetskij, Y., P. Kalenyuk, M. Kopach, and A. Solomko. “The Nonlocal Boundary Value Problem With Perturbations of Mixed Boundary Conditions for an Elliptic Equation With Constant Coefficients. II”. Carpathian Mathematical Publications, Vol. 12, no. 1, June 2020, pp. 173-88, doi:10.15330/cmp.12.1.173-188.